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Temperature-dependent angle-resolved photoemission spectroscopy (ARPES) was carried out on single-crystalline EuB6 samples. By measuring ARPES spectra in an extended Brillouin zone, a B 2p hole pocket centered at the X point is clearly observed, thus proving the semimetallic nature of EuB6. Below the Curie temperature TC, ARPES spectra show two B 2p bands of which separation is due to an exchange interaction between local Eu 4f and itinerant B 2p electrons. The exchange splitting becomes smaller as the temperature increases and disappears well above TC. Additionally, a diffuse structure near the Fermi level survives just above TC. Such behavior is well described by Monte Carlo simulations of a Kondo lattice model, thus supporting the formation of magnetic polarons in EuB6, which accounts for the resistivity upturn near above TC when lowering the temperature., Comment: 6 pages, 5 figures

Author(s): Xu, Y; Kotta, EC; Song, MS; Kang, BY; Lee, JW; Cho, BK; Liu, S; Yilmaz, T; Vescovo, E; Denlinger, JD; Miao, L; Wray, LA | Abstract: SmB6 is a strongly correlated material that has been attributed as a topological insulator and a Kondo insulator. Recent studies have found the topological surface states and low temperature insulating character to be profoundly robust against magnetic and nonmagnetic impurities. Here, we use angle resolved photoemission spectroscopy to chart the evolution of topologically linked electronic structure features versus magnetic doping and temperature in Sm1-xMxB6(M=Eu,Ce). Topological coherence phenomena are observed out to ∼30% Eu and 50% Ce concentrations that represent extreme nominal hole and electron doping, respectively. Theoretical analysis reveals that a recent redesignation of the topologically inverted band symmetries provides a natural route to reconciling the persistence of topological surface state emergence even as the insulating gap is lost through decoherence.

A well-known feature in transport data of the topological Kondo insulator SmB6 is the sign change in the Hall coefficient at 65 K. Carriers in SmB6 are known to be negative, but above 65 K, the Hall sign suggests that the carriers are positive. Here, we extend Hall measurements up to 400 K and observe that the Hall coefficient changes back to the correct (negative) sign at about 305 K. We interpret the anomalous sign of the Hall coefficient in the context of skew scattering arising from the strong correlations between the f and d electrons. At energy scales where the gap is closed, the number of d electrons in resonance with the f electrons at the Fermi energy varies. When a large proportion of d and f electrons are in resonance, skew scattering is dominant, leading to the observation of the positive sign, but when fewer are in resonance, conventional scattering mechanisms dominate instead.

The frustrated magnet has been regarded as a system that could be a promising host material for the quantum spin liquid (QSL). However, it is difficult to determine the spin configuration and the corresponding mechanism in this system, because of its geometrical frustration (i.e., crystal structure and symmetry). Herein, we systematically investigate one of the geometrically frustrated magnets, the TbB4 compound. Using resonant soft x-ray scattering (RSXS), we explored its spin configuration, as well as Tb's quadrupole. Comprehensive evaluations of the temperature and photon energy/polarization dependences of the RSXS signals reveal the mechanism of spin reorientation upon cooling down, which is the sophisticated interplay between the Tb spin and the crystal symmetry rather than its orbit (quadrupole). Our results and their implications would further shed a light on the search for possible realization of QSL.

We investigate the roles of disorder on low-temperature transport in SmB$_6$ crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arises from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. We consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of SmB$_6$., 22 pages, 4 figures

After the theoretical prediction that ${\mathrm{SmB}}_{6}$ is a topological Kondo insulator, there has been an explosion of studies on the ${\mathrm{SmB}}_{6}$ surface. However, there is not yet an agreement on even the most basic quantities such as the surface carrier density and mobility. In this paper, we carefully revisit Corbino disk magnetotransport studies to find those surface transport parameters. We first show that subsurface cracks exist in the ${\mathrm{SmB}}_{6}$ crystals, arising both from surface preparation and during the crystal growth. We provide evidence that these hidden subsurface cracks are additional conduction channels, and the large disagreement between earlier surface ${\mathrm{SmB}}_{6}$ studies may originate from previous interpretations not taking this extra conduction path into account. We provide an update of more reliable magnetotransport data than the previous one (S. Wolgast et al., Phys. Rev. B 92, 115110) and find that the orders-of-magnitude large disagreements in carrier density and mobility come from the surface preparation and the transport geometry rather than the intrinsic sample quality. From this magnetotransport study, we find an updated estimate of the carrier density and mobility of $2.71\ifmmode\times\else\texttimes\fi{}{10}^{13}$ (1/${\mathrm{cm}}^{2}$) and 104.5 (${\mathrm{cm}}^{2}$/$\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{sec}$), respectively. We compare our results with other studies of the ${\mathrm{SmB}}_{6}$ surface. By this comparison, we provide insight into the disagreements and agreements of the previously reported angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and magnetotorque quantum oscillations measurements.

2D correlation analysis (2DCOS) and principal component analysis (PCA) have been performed on temperature-dependent Raman spectra from 12 K to 300 K of SmB6 single crystals grown from Boron with a purity of 99.9% (3N) or 99.9999% (6N). Significant differences in the Raman spectra of SmB6(3N) and SmB6(6N) are observed. The Raman active T2g, Eg, and A1g modes of SmB6(3N) are broader and shift to higher wavenumbers compared with those of SmB6(6N). The results of 2DCOS and PCA on temperature-dependent Raman spectra of SmB6(3N) and SmB6(6N) give us further information. We find that all the T2g, Eg, and A1g modes of SmB6(6N) are single bands, while those of SmB6(3N) show all doublet features. The appearance of multiple bands in Raman vibrational modes of SmB6(3N) sample implies that the structure of boron octahedra of the SmB6(3N) has lower symmetry than that of SmB6(6N). Our findings in 2DCOS and PCA of the Raman spectra strongly support the differences in the behavior of SmB6(3N) and SmB6(6N).

Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm1-xMxB6 (M=Ce, Eu), using angle-resolved photoemission spectroscopy (ARPES) and complementary characterization techniques. Remarkably, topologically nontrivial bulk and surface band structures are found to persist in highly modified samples with up to 30% Sm substitution, and to coexist with antiferromagnetism in the case of Eu doping. The results are interpreted in terms of a hierarchy of energy scales, in which surface state emergence is linked to the formation of a direct Kondo gap, while low temperature transport trends depend on the indirect gap., Main text with 4 figures, and supplementary PDF

The interplay of charge, spin, orbital and lattice degrees of freedom has recently received great interest due to its potential to improve the magnetocaloric effect (MCE) for the purpose of magnetic cooling applications. Here we propose a new mechanism for a giant inverse MCE in rare-earth tetraborides, especially for Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0). For x = 0.0, 0.5, and 1.0, the maximum entropy changes of the giant inverse MCE are found to be 22.7 J/kgK, 19.6 J/kgK, and 19.0 J/kgK with critical fields of 25 kOe, 40 kOe, and 50 kOe, respectively. It is remarkable that such a giant MCE is realized, even when applying a low magnetic field, which enables a field-tuned entropy change and brings about a significant advantage for several applications. For all compounds, we have systematically studied how the entropy changes as a function of the field and temperature and investigated their correlation with consecutive double transitions, i.e., the magnetic dipolar order at T = TN and the quadrupolar order at T = TQ (TQ < TN). We found that the maximum entropy change occurs at T = TQ and the critical field associated with the meta-magnetic transition, which is in good agreement with the experimental data. Thus, we elucidate that this unique behaviour is attributed to the strong coupling between magnetic dipoles and quadrupoles in the presence of strong spin-orbit coupling and geometric frustration. Our work offers new insights into both the academic interest of multipolar degrees of freedom in magnetic materials and the discovery of giant MCE with various applications for magnetic cooling systems., 25 pages, 12 figures

Emergence of anomalous weak ferromagnetism in geometrically frustrated magnetic system of R 1− x Y x B 4 ( R = Tb and Dy) is quite interesting [Phys. Rev. B 91 , 024414 (2015)]. In order to study the weak ferromagnetism in other compounds, the magnetic properties of R 1− x Y x B 4 ( R = Sm, Gd, and Er) were investigated. The anomalous weak ferromagnetism was also found to occur in all compounds, R 1− x Y x B 4 ( R = Sm, Gd, and Er). The weak ferromagnetism is revealed in the magnetic easy plane, (001), for R 1− x Y x B 4 ( R = Sm and Gd) and along the magnetic easy axis, [001], for R 1− x Y x B 4 ( R = Er). The magnitude of saturated magnetic moment at T = 2 K shows a strong dependence on Y-concentration, i.e., the maximum magnitude occurs at x ≈ 0.3 for all compounds. The general magnetic properties of weak ferromagnetism is quite similar to the weak ferromagnetism in R 1− x Y x B 4 ( R = Tb, Dy). Therefore, it is experimentally demonstrated that the emergence of weak ferromagnetism, induced by non-magnetic substitution in a magnetically frustrated system, is the universal feature in R B 4 compounds.

Samarium hexaboride (SmB6), which lies in the mixed valence regime in the Anderson model, has been predicted to possess topologically protected surface states. The intensive investigations on SmB6 have brought up the long standing questions about the discrepancy between the surface and bulk electronic properties in rare-earth compounds in general. Here, we investigate and eventually clarify this discrepancy in the particular case of SmB6 by the photoemission core-level spectra. We focus on the change in both Sm and B states depending on time, temperature, probing depth, and surface termination on the cleaved (100) surface. Our spectra show that the unusual time-dependent change in the Sm valence occurs within a period of hours, which is not related to the adsorption of residual gases. Moreover, we observe a reduction of the surface feature in the B and Sm states on the same timescale accompanied by the formation of a subsurface region. Thus, it indicates the relatively slow charge redistribution between the surface and subsurface regions. Our findings demonstrate that the f states is strongly involved in the surface relaxation.

Weyl semimetal is a topologically non-trivial phase of matter with pairs of Weyl nodes in the k-space, which act as monopole and antimonopole pairs of the Berry curvature. Two hallmarks of the Weyl metallic state are the topological surface state called the Fermi arc and the chiral anomaly. It is known that the chiral anomaly yields anomalous magnetotransport phenomena. In this study, we report the emergence of the type II Weyl semimetallic state in the geometrically frustrated non-collinear antiferromagnetic Shastry-Sutherland lattice (SSL) GdB4 crystal. When we apply magnetic fields perpendicular to the non-collinear moments in the SSL plane, Weyl nodes are created above and below the Fermi energy along the M-A line (τ-band) because the spin tilting breaks the time-reversal symmetry and lifts band degeneracy while preserving C 4 z or C 2 z symmetry. The unique electronic structure of GdB4 under magnetic fields applied perpendicular to the SSL gives rise to a non-trivial Berry phase, detected in de Haas-van Alphen experiments, and chiral anomaly–induced negative magnetoresistance. The emergence of the magnetic field–induced Weyl state in the SSL presents a new guiding principle to develop novel types of Weyl semimetals in frustrated spin systems.

Paramagnetic heavy fermion insulators consist of fully occupied quasiparticle bands inherent to Fermi liquid theory. The gap emergence below a characteristic temperature is the ultimate sign of coherence for a many-body system, which in addition can induce a non-trivial band topology. Here, we demonstrate a simple and efficient method to compare a model study and an experimental result for heavy fermion insulators. The temperature dependence of the gap formation in both local moment and mixed valence regimes is captured within the dynamical mean field (DMFT) approximation to the periodic Anderson model (PAM). Using the topological coherence temperature as the scaling factor and choosing the input parameter set within the mixed valence regime, we can unambiguously link the theoretical energy scales to the experimental ones. As a particularly important result, we find improved consistency between the scaled DMFT density of states and the photoemission near-gap spectra of samarium hexaboride (SmB6).

Author(s): He, H; Miao, L; Augustin, E; Chiu, J; Wexler, S; Breitweiser, SA; Kang, B; Cho, BK; Min, CH; Reinert, F; Chuang, YD; Denlinger, J; Wray, LA | Abstract: The compound SmB6 is the best established realization of a topological Kondo insulator, in which a topological insulator state is obtained through Kondo coherence. Recent studies have found evidence that the surface of SmB6 hosts ferromagnetic domains, creating an intrinsic platform for unidirectional ballistic transport at the domain boundaries. Here, surface-sensitive x-ray absorption (XAS) and bulk-sensitive resonant inelastic x-ray scattering spectra are measured at the Sm N4,5 edge, and used to evaluate electronic symmetries, excitations, and temperature dependence near the surface of cleaved samples. The XAS data show that the density of large-moment atomic multiplet states on a cleaved surface grows irreversibly over time, to a degree that likely exceeds a related change that has recently been observed in the surface 4f orbital occupation.

Half-filled electron systems, even with the maximized spin angular moment, have been given little attention because of their zero-orbital angular moment according to Hund's rule. Nevertheless, there are several measurements that show evidence of a nonzero orbital moment as well as spin-orbit coupling. Here we report for the first time the orbital order in a half-filled 4f-electron system GdB_{4}, using the resonant soft x-ray scattering at Gd M_{4,5}-edges. Furthermore, we discovered that the development of this orbital order is strongly coupled with the antiferromagnetic spin order. These results clearly demonstrate that even in half-filled electron systems the orbital angular moment can be an important parameter to describe material properties, and may provide significant opportunities for tailoring new correlated electron systems.

Kondo insulator $\mathrm{Sm}{\mathrm{B}}_{6}$ has attracted attention because it can realize new topological phenomena driven by the interplay between strong correlation effect and topology. However, its topological nature is still under debate. To examine the topological aspect, we demonstrate the nonmagnetic La and magnetic Ce doping dependence of the resistance of $\mathrm{Sm}{\mathrm{B}}_{6}$. Moreover, the resistance ratios of different thicknesses are analyzed to confirm the surface contribution. Lightly doped La samples show a purely conducting surface region at low temperature, whereas the lightly doped Ce samples do not have any conducting region at low temperature. Furthermore, based on the analysis of the electrical transport data of $\mathrm{S}{\mathrm{m}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}{\mathrm{B}}_{6}$ $(0.0\ensuremath{\le}x\ensuremath{\le}1.0)$, an electronic phase diagram was found, composed of four regions: region I $(0.0\ensuremath{\le}x\ensuremath{\le}0.06)$, II $(0.1\ensuremath{\le}x\ensuremath{\le}0.15)$, III $(x\ensuremath{\approx}0.2)$, and IV $(0.25\ensuremath{\le}x\ensuremath{\le}1.0)$. Region I is characterized by the presence of conducting surface states, region II is characterized by the insulating phase due to the $d\ensuremath{-}f$ hybridization gap without the conducting surface state, region III is characterized by the disappearance of the $d\ensuremath{-}f$ hybridization gap and the existence of valence fluctuation, and region IV is a typical metallic state.

To finally resolve the controversial issue of whether or not the electronic structure of YbB_{6} is nontrivially topological, we have made a combined study using angle-resolved photoemission spectroscopy (ARPES) of the nonpolar (110) surface and density functional theory (DFT). The flat-band conditions of the (110) ARPES avoid the strong band bending effects of the polar (001) surface and definitively show that YbB_{6} has a topologically trivial B 2p-Yb 5d semiconductor band gap of ∼0.3 eV. Accurate determination of the low energy band topology in DFT requires the use of a modified Becke-Johnson exchange potential incorporating spin-orbit coupling and an on-site Yb 4f Coulomb interaction U as large as 7 eV. The DFT result, confirmed by a more precise GW band calculation, is similar to that of a small gap non-Kondo nontopological semiconductor. Additionally, the pressure-dependent electronic structure of YbB_{6} is investigated theoretically and found to transform into a p-d overlap semimetal with small Yb mixed valency.

We present a detailed study on the temperature-dependence of the Sm 4 f states in mixed-valent samarium hexaboride (SmB 6 ) which, as a consequence of the hybridization with the conduction band states, largely determine the physical properties of the system. By comparing soft X-ray photoemission spectra of SmB 6 with those of Sm 0.35 La 0.65 B 6 , we discuss the apparent differences and similarities of the two systems. Based on our analyses we show that the 4 f spectra of SmB 6 can be separated into two independent spectral contributions with the 4 f peak maxima at different binding energies, representing two different electronic 4 f configurations existing in the system.

Strong spin-orbit interaction and time-reversal symmetry in topological insulators generate novel quantum states called topological surface states. Their study provides unique opportunities to explore exotic phenomena such as spin Hall effects and topological phase transitions, relevant to the development of quantum devices for spintronics and quantum computation. Although ultrahigh-vacuum surface probes can identify individual topological surface states, standard electrical and optical experiments have so far been hampered by the interference of bulk and quantum well states. Here, with terahertz time-domain spectroscopy of ultrathin Bi2Se3 films, we give evidence for topological phase transitions, a single conductance quantum per topological surface state, and a quantized terahertz absorbance of 2.9% (four times the fine structure constant). Our experiment demonstrates the feasibility to isolate, detect and manipulate topological surface states in the ambient at room temperature for future fundamental research on the novel physics of topological insulators and their practical applications.

We investigate temperature-dependent carrier dynamics of InAs crystal by using reflection-type terahertz time-domain spectroscopy, particularly with a recently developed emitter-sample hybrid structure. We successfully obtain the optical conductivity in a terahertz frequency of bulk InAs whose dc conductivity is in the range of 100-150 Ωsup-1/sup cmsup-1/sup. We find that both real and imaginary parts of the optical conductivity can be fit well with the simple Drude model, and the free-carrier density and the scattering rate obtained from the fit are in good agreement with corresponding values obtained by using other techniques, such as the Hall measurement and the dc-resistivity measurement. These results clearly demonstrate that the proposed technique of adopting the emitter-sample hybrid structure can be exploited to determine temperature-dependent optical constants in a reflection geometry and hence to investigate electrodynamics of bulk metallic systems.

Strong spin-orbit interaction and time-reversal symmetry in topological insulators generate novel quantum states called topological surface states. Their study provides unique opportunities to explore exotic phenomena such as spin Hall effects and topological phase transitions, relevant to the development of quantum devices for spintronics and quantum computation. Although ultrahigh-vacuum surface probes can identify individual topological surface states, standard electrical and optical experiments have so far been hampered by the interference of bulk and quantum well states. Here, with terahertz time-domain spectroscopy of ultrathin Bi₂Se₃ films, we give evidence for topological phase transitions, a single conductance quantum per topological surface state, and a quantized terahertz absorbance of 2.9% (four times the fine structure constant). Our experiment demonstrates the feasibility to isolate, detect and manipulate topological surface states in the ambient at room temperature for future fundamental research on the novel physics of topological insulators and their practical applications.

The discovery of topologically non-trivial states in band insulators has induced an extensive search for topological phase in strongly correlated electron systems. In particular, samarium hexaboride (SmB$_6$) has drawn much attention as it might represent a new class of condensed matter called topological Kondo insulator. Kondo insulators (KI) can have non-trivial Z$_2$ topology because the energy gap opens at the Fermi energy ($E_F$) by hybridization between an odd- parity renormalized $f$ band and an even- parity conduction $d$ band. However, the characteristics of SmB$_6$ deviate from the conventional KI because its gap is insensitive to doping and pressure. Thus, it is unclear what makes the gap of SmB$_6$ different from that of KI, and how the band inversion features occur. In this manuscript, we demonstrate the importance of charge fluctuations in SmB$_6$. Our angle-resolved photoemission spectroscopy (ARPES) results reveal that with decreasing temperature the bottom of the $d$-$f$ hybridized band at the $\bar{\text{X}}$ point gradually shifts from below to above $E_F$. This shift accompanies a redistribution of spectral weight from the upper to the lower quasiparticle band, and reflects the change in both carrier density and the number of localized $f$ electron. Moreover, because this hybridized band is predicted to have odd parity and to induce a non-trivial $Z_{2}$ topology, we compare our ARPES spectra with theoretical surface band structures and find signatures of topological surface states. Therefore, our results indicate that SmB$_6$ is a charge fluctuating topological insulator having the odd-parity hybridized band above $E_F$ at the $X$ point. This demonstrates that the charge fluctuations do not only drive the insulating bulk phase but also the non-trivial topological phase., 12 pages, 5 figures

This paper investigates the magnetic and electronic properties of a single-crystal SmB4. Two magnetic phase transition temperatures at TN=25 K and Tm≈7 K were revealed by the temperature-dependent magnetization for an applied magnetic field perpendicular to the c-axis. Magnetic transition at TN=25 K is caused by the antiferromagnetic order with Sm3+ moments that lie on the ab-plain. However, the antiferromagnetic transition temperature was not observed in a temperature-dependent magnetization for an applied magnetic field along the c-axis. The second transition occurring at Tm≈7 K was observed for directions both parallel and perpendicular to the c-axis and was suppressed by applying magnetic fields. Interestingly, the second transition exhibited local maximum behavior, implying the second order phase transition. Furthermore, temperature-dependant resistivity showed a magnon gap feature below the second magnetic transition temperature T